What shall i do about my friend ?

I love my friend to pieces but she's acting a little bit like a tart lately. We're in year 8 and me and all my other friends hang around with Year 8 lads but she hangs around with all the Year 11 lads. She wears thongs and like, bends down in front of them and everything. And me and my friends are like :| she's got like all their numbers and talks to them on msn and stuff. What shall i do ? x